Jib Boom

Next silly question. I’ve been trying to get my head around this for a couple of days but a problem with two or more variables does a number on my old brain. What effect does a bendy jib boom have? If any? Backstay tension? Sail twist? Mast rake? Mast bend? All of the above?

Do you mean a bendy boom (ie flexible) or a bent boom?

That is a very interesting question indeed, but I’m not sure you’ve et enough parameters about the way it bends (i.e. how it is rigged) to make it terribly meaningful yet.:graduate:

I wondered if this was going to get any responses. I mean a flexible boom with a topping lift, rigged to pivot at about 25%. Kind of the standard for IOM and USOM and probably lots more.

interesting, a flexible boom… it might well have some potential to fine-tune sail shape, but it would likely require much in the way of little control lines, and it would certainly be complicated to adjust under sail… however, if someone could convince me of the benefits…

OK, let me try a different tack. I have a USOM with the normal jib rigging. I was using a CF arrow shaft for the boom. I have replaced the boom with a piece of CF fishing rod. It is much thinner, flexible and tapered. Is the flex going to affect the way the boat sails?

There are different degrees of “bendy”. If it is too limber then you’ll have a loose leech or possibly a loose jib stay. You will answer your own question when you sail the boat. If the leech flutters or sags far off to leeward then the boom is too bendy. If there is too much sag in the leech it will make a curve that messes up the slot that you are trying to optimize.

There may be some substance in having a resilient boom. Maybe it would open the slot in gusts and therefore constitute an advantage. The main boom might be worth some attention, in this regard, as well. The concept is worth experimentation.

Would you even notice the loose leach? If the boom was stiff enough to only bend in a puff the leach might slacken momentarily and then tighten and you might not see it. If the jib leach was to slacken in a puff could that cause some weather helm as the jib is depowered?

Don, I would argue that you do not want a boom that flexes. The problem is that the flex absorbes wind energy in a gust. . .energy that should be used to accelerate the boat. A stiff boom is better.

If you are using a boom that has its deck attachment set a couple of inches back from the tack, then the rigging acting on the boom goes from the backstay which not only bends the mast, but also tensions the forestay. Forestay tension pulls up on the front of the boom causing a downward pressure on the jib clew and tensioning the jib leach. Sometimes the backstay/forestay tension required can overtighten the jib leach. Enter the jib boom topping lift. This is adjusted to counter the pull of the forestay, allowing you to set and control the jib leach twist. A flexible boom would interfer with the forces involved in this ‘triangle of forces’.


The main reason you want the jib boom to be as stiff as possible is that you do not want the foot of the jib to get fuller in puffs. That is the opposite of what you want to see happening. More wind in the sail - more pressure load - more bending - fuller jib foot.

fuller jib foot - more power in the jib at a time when you would rather see the jib depowering or at least see it not gain power.

I’m pretty much convinced that a bendy boom is bad, mostly because of the unpredicability of the thing. I’ve thought this from the beginning but I had these twinges of uncertanty, thats why I posted the question. Even with your input there is still some grey areas. I can see the foot getting shorter and fuller as the boom bends but the leach is opening at the same time-wouldn’t they cancel each other somewhat? I would think that it would be nice to have the jib power up in a puff to counter the increase weather helm. Actually maybe a bendy main boom would be better. Does anyone have any thoughts about a bendy jib booms effect on the main sail. I originally thought that the backstay might slacken but since the forces are tensioning the backstay and the windward shroud I’ve convinced myself that all the effects would be in the jib. Everything concerning the jib would slacken leach, luff and foot. I suppose that depending on the rig tensions that this slack could work it’s way into the backstay and cause the mast to straighten a bit(assuming it was bent). See, to many variables and my brain clogs up. Deep down I think there could be some gains at some points but predicting and controlling it would be extemely tough.
Thanks for the input

Hi Don,
I’ve been thinking about this one too.

The effect of a bendy jib boom would be similar to having a spring in the jib club swivel. Unless taken to extremes, the foot length change is going to be minimal. The leech and luff lengths are fixed by the topping lift and jib stay. The camber might increase very minimally.

What will change is the effective length of the jib stay resulting in a slackening of the backstay leading to increased camber in the main. Just what you don’t want in a puff.

I think you’re right. Not a good thing…